The FBI has warned US officials of a possible al-Qaeda plot against the subway and transport grid in New York.
Extra police are stationed around major hubs in the city on the eve of the Thanksgiving holiday.
Bot the the FBI and the Department of Homeland Security acknowledged on Wednesday that the threat was "plausible", but unsubstantiated.
Officials said the US terror alert level has not been raised.
ABC TV News quoted unnamed law enforcement sources as saying that the attack under discussion targeted Penn Station, a major subway and rail hub in Manhattan.
According to ABC News, the plot involved a "Madrid-like attack" on Penn Station - a reference to the bombing of Spanish trains in March 2004 that killed 191 people.